iTunes/iPhone might be defect

Hi guys I have a question. I just bought myself a iPhone and already have problems. I want to put music on the phone though iTunes won't give me the oppurtunity to put it there. At first when I synced my phone with iTunes I was able to put music on the phone though now after using it for some time it won't let me put music on the phone. You all probably know that iTunes 11 has this button/folder which is named ''on this iphone'' on which you have to click in order to put music and stuff and iOS devices. so my problem is that it is missing and I can not get iTunes to let me do anything with the phone via computer. What shall I do now?